PROJECT SUMMARY/ABSTRACT In response to damage, pathological cardiac remodeling frequently manifests as myocardial hypertrophy, fibrosis, contractile dysfunction and/or increased arrhythmia vulnerability. Myofibroblasts (Mfbs) are not normally present in the working myocardium but accumulate in large numbers in the injured heart. Hypersecretion of collagen and paracrine factors from activated Mfbs results in the formation of electrically insulating septa and ionic remodeling of cardiomyocytes (CMs), respectively, promoting arrhythmia. Besides their barrier and paracrine function, preliminary experiments using intravital voltage imaging and immunohistochemistry, suggest the intriguing possibility that Mfbs influence the functional properties of fibrotically remodeled myocardium via direct electrotonic interactions with surrounding CMs via gap junctions composed of cell-to-cell Connexin(Cx)43 subunits. Preliminary mouse modeling data also revealed that chronic systemic infusion of ?-adrenergic or angiotensin-II receptor agonists, as well as acute myocardial infarction (MI) result in Mfb-restricted misexpression of Cx43, a stretch-induced profibrotic cytokine Ccl2 (MCP-1) and profibrotic matricellular Periostin (Postn) coincident with pathological remodeling. Likewise, in patients with acute MI or pressure overload, POSTN and MCP-1 are robustly upregulated and Cx43 (GJA1) distribution is perturbed. Cx43, the principal gap junction protein responsible for action potential propagation in ventricles, is often mis-expressed in hypertrophied and ischemic patient hearts. We and others have demonstrated that Cx43 is also present in the pathological Mfb population, and additional preliminary data revealed Ccl2 is transcribed in Mfbs (whereas its receptor Ccr2 is in CMs). Endogenous Postn functionally promotes cardiac fibrosis/ventricular stiffness, as surviving Postn nulls exhibit less fibrosis and better function after MI. Moreover, we generated a unique 3.9kb-driven Postn enhancer reporter line that drives Cre-recombinase expression only in Mfbs in injured hearts (designated Postn-Cre). Thus, this is a most useful tool for genetic manipulation of cardiac activated injury-site Mfbs. However, the actual role that electrical coupling in Postn-expressing Mfb-lineage plays and the mechanism underlying Mfb-CM interactions during pathological remodeling remain unknown. Thus, three interrelated aims are proposed to examine these preliminary data. Aim 1 will test the hypothesis that uncoupling Mfb-CM electrical signaling via Mfb-restricted cKO of Cx43 prevents adverse cardiac remodeling. Aim 2 will test whether Mfb-restricted Cx43 cKO offers arrhythmia protection. Aim 3 will determine if Mfb-restricted loss of the Ccl2 cytokine ameliorates adjacent CM contractile dysfunction.